This invention relates to a magnetic recording medium of the type consisting of a base in the form of a tape or sheet coated with a magnetic layer made of a magnetic powder dispersed in a binder. More particularly, the invention concerns a magnetic recording medium of the type defined above which excels in electromagnetic properties and is suitable for use with an automatic magnetic tape stopping system.
Compact household video tape recorders, or home video units, using the 1/2-in.-wide magnetic recording medium (magnetic tape) have rapidly spread in recent years. The recording medium to be used comprises a nonmagnetic base in the form of a tape or sheet and a magnetic layer made of a magnetic powder dispersed in a resin binder and formed thereon. The magnetic recording-reproducing units, intended primarily for use at home, are designed and built to be simple to operate.
For an example, the home video unit is equipped with means of automatically stopping the magnetic tape when the tape end has been reached during the run, such as for recording, playback, fast forward, or fast rewind motion. The principle of the mechanism is as follows. The magnetic tape is provided with short lengths of transparent leader tape spliced to the both ends. The video unit is equipped with means to direct a beam of light to the running magnetic tape during the operation and detect the light transmitted through the leader tape portion as an indication of the arrival of the starting or finishing tape end. Upon detection of the transmitted light the system brings the magnetic tape to a stop. The means for transmitted light detection would in some cases respond to light from sources other than the video unit. To avoid this, the detector is designed to remain inoperative when the quantity of light transmitted through the tape is less than a certain reference level. In this country the reference level, or light transmittance of the magnetic tape body, should be 0.1% or below as measured in conformity with the procedure of Japanese Industrial Standards C-6280.
For more accurate operation of the automatic stopping system based on the above principle, it is essential that the magnetic tape body be sufficiently light-shielding. Apparently, the end may be achieved, for example, by
(1) making the magnetic layer thick enough to keep light from traveling through the magnetic tape body, PA1 (2) allowing the magnetic layer to contain a large proportion of a light-shielding substance so that, even if the layer itself is thin, light transmission through the magnetic tape body can be prevented, or PA1 (3) employing a magnetic powder which transmits little light. Of these methods, (1), or the adoption of a thicker magnetic layer, has a drawback of an increased loss of recording efficiency due to the influence of an antimagnetic field. Also, the magnetic tape for the home video unit that must be compact in construction cannot have a greater thickness (than the usual level of about 15 to 20 .mu.m). If the magnetic layer is to be thicker, the base film that supports the layer will have to be thinner accordingly. This will result in a sacrifice of the strength of the magnetic tape and frequent break or deformation of the tape.
The next method (2) possibly involves the inclusion of a large proportion of carbon powder or the like into the magnetic layer. In order to prevent the light transmission, the layer must contain at least about 10% by weight of carbon powder or the like on the basis of the weight of .gamma.-Fe.sub.2 O.sub.3 or other magnetic powder. Magnetic tape with such a large carbon powder content presents a problem of very poor electromagnetic properties.
The last resort (3) is the replacement of .gamma.-Fe.sub.2 O.sub.3 by a less light-transmissive substance, perhaps Fe.sub.3 O.sub.4. However, the latter is undesirable as a magnetic powder for magnetic recording medium because of its instability to temperature changes and susceptibility to oxidation.
Video tapes today are required to have higher density and be longer-playing than heretofore. Efforts are therefore being made to employ finer magnetic powder and greater reduction of the tape thickness. The trends are thus more and more adverse to the adoption of the automatic stopping system that depends on the light transmission through the tape. For instance, further size reduction of the cobalt-containing .gamma.-Fe.sub.2 O.sub.3 currently used as the magnetic powder for video tapes would make the tapes more light-transmissive. The use of thinner magnetic tape would add to the transmissivity to make the matter worse.